A wide range of doses of the potent hallucinogen LSD (0.05 - 2.0 mg/kg) were tested for effects on striatal tyrosine hydroxylase. When assayed at saturating and subsaturating cofactor concentrations, enzyme activity was not altered 30 to 60 min after treatment. Based on these results, LSD may increase the rate of dopamine synthesis via a mechanism other than a kinetic change in tyrosine hydroxylase.